Communications system frequently include a plurality of network nodes which are coupled to access nodes, e.g., base stations, through which end nodes, e.g., wireless terminals, are coupled to the network.
In a wireless communication system, effective utilization of the network resources, such as spectrum, requires a base station to efficiently assign wireless terminals for reception and transmission in available traffic channels. Often, to enhance the resource utilization, a dedicated traffic assignment channel, or channel of that nature, is implemented in a wireless communication system. Frequently, the problem in designing traffic assignment channel arises from the fact that assignment channel has to reach every wireless terminal for the assignment to be effective. This universal accessibility, even for wireless terminals residing at low signal-to-noise ratios, requires that the assignment channel to be sent at relatively high power. Sometimes this requirement incurs unnecessary high power to be transmitted at the access nodes, e.g base stations, especially when there are only a few low SNR wireless terminals, and degrades the overall efficiency of the wireless communication system.
To make assignment more power efficient, some wireless communication systems, e.g. Flash-OFDM system, further introduce a special dedicated assignment channel for wireless terminals at low SNRs. Because the bad SNR mobiles are relatively few, the number of bits need to be transmitted for assignment is small and therefore optimal coding only requires a few symbols, in which case sufficiently high power can be assigned to guarantee the reach of the few wireless terminals at low SNRs. Now, since the few number of modulated symbols spread sparsely in time and frequency, the dedicated assignment channel for low SNR wireless terminals can be now more susceptible to impairment prevalent in wireless communication, e.g., frequency selective fading due to multi-path reception and/or fast fading due to mobility of the wireless terminal. Channel fading causes large variations of the channel gain experienced on the few modulated symbols of the dedicated assignment channel for low SNR wireless terminals. While some of the modulated symbols experience much stronger path gain than the mean, other modulated symbols can be deeply faded. Therefore, to make the dedicated assignment channel for low SNR wireless terminal effective and efficient, it is important for the communication method to be robust against fading. In particular, the communication method needs to be robust against the erasure of some portion of the symbols. Often, communication methods designed for Additive White Gaussian Noise (AWGN) channels do not suffice.
In view of the above discussion, it should be appreciated that there is a need for new and improved ways of communicating dedicated assignment signals for low SNRs wireless terminals which are more robust against large variation of channel gains due to e.g., frequency selective fading and fast fading in time.